With recent revolutionary advances in electronic technologies, the integration of electronic circuits and the digitalization of signal processing have become common techniques. In such circumstances, the miniaturization/integration of continuous-time system filters essential for analog signal processing has been developed in the form of active RC filters. In particular, the use of higher frequency bands being accelerated in line with recent digitalization requires taking up the challenge of assuring high frequency characteristics. Therefore, signal processing in higher frequency bands has been developed to achieve high frequency characteristics and high integration in active RC filters. In terms of noise problems involved in signal processing, continuous-time system filters also have advantage in intermediate or low frequency characteristics, and developments for achieving active RC filters usable in intermediate or low frequency bands and integration thereof have been made.
A second-order active circuit as a fundamental element of such an active RC filter includes a Sallen-Key circuit using a positive-phase-sequence amplifier, a circuit using a single amplifier and a circuit using a gyrator.
In the Sallen-Key circuit, a positive feedback characteristic is caused at the polar frequency (center frequency), and the sensitivity of Q to variations in associated elements is extremely high. While the single-amplifier type circuit based on multiple-feedback can stably achieve a high Q-value, it has a feed gain including the open loop gain of an operational amplifier at the polar frequency. While the transistor gyrator circuit can also stably obtain a high Q-value in a high frequency range without difficulties, a positive feedback characteristic is caused at the polar frequency, and thus a desirable low-noise performance is hardly obtained.
As above, even though the conventional active RC filters based on the second-order active circuits, such as the Sallen-Key circuit using a positive-phase-sequence amplifier, the single-amplifier type circuit and the gyrator circuit, can conveniently obtain a high Q-values, all of them cannot maintain a negative feedback loop at the polar frequencies of the second-order active circuit. Thus, these RC filters still involve a problem of difficulty in sufficiently reducing noises.
Due to the difficulty in striking a balance between high Q-value and low noise performance in a high frequency range, any active RC filter usable in a high frequency range has not been put to practical use up to now. In the practical design of filter circuits for use in a high frequency range, it has no choice but to employ an active coil (chip inductor) and externally combine it therewith. In particulate, this constitutes an adverse factor against achievement of small-sized monolithic ICs for use in a high frequency range.
Thus, there is still the need for facilitating integration between various filter circuits including an inductance and other circuits to provide downsized circuitries.
It is therefore an object of the present invention to provide an active RC signal processing circuit having a transfer function capable of changing a negative feedback loop gain such that a transmission gain is reduced at a value equal to or less than a forward gain over the entire frequency range, to achieve a low noise characteristic.